1. Field of the Invention
The present invention relates to temperature sensors for heated rollers in sheet conveyors and, more particularly, to the prevention of lint buildup on heat sensors attached to flatwork ironers.
2. Description of the Prior Art
Flatwork ironers are widely used in laundries for hotels, hospitals, restaurants and related institutions to dry and iron linen items such as sheets, pillowcases, tablecloths, towels, napkins, etc. In a typical embodiment damp flatwork is fed onto canvas conveyor ribbons and transported under compression rollers for initial stretching, smoothing and flattening, then between return ribbons and an upper rotating cylinder or roller, generally gas heated, where the adjacent side of the flatwork is dried and further pressed. The fabric is then conveyed to a second heated rotating cylinder where the opposite side of the material is dried and pressed. The flatwork is transported by the return ribbons at a speed slightly faster than the rotating speed of the heated cylinders to produce a sliding contact that produces calendared or glossy finish on the ironed flatwork.
Temperatures up to nearly 400.degree. F. and transport speeds of up to about 150 feet per second pose severe safety hazards to operating personnel. As a consequence flatwork ironer components are arranged to displace the dangerous heated rotating cylinders to locations remote from operator interaction. In addition guards and shrouds are placed around the equipment to prevent or discourage accidental insertion of hands and arms into proximity with the heated cylinders.
Because different fabric requirements and production rates demand selective temperature control, sensors are mounted against each of the heated cylinders to provide continual temperature monitoring, and additional high temperature limit switches are also mounted against each heated cylinder to automatically shut down the heater when excessive temperature is sensed, to prevent scorching of flatwork or burning of the return ribbons.
Both the heat monitor/control switch and the limit switch typically have brass shoes spring-loaded to press against a portion of the rotating cylinder surface not in contact with the flatwork.
The nature of flatwork ironing produces a great deal of lint requiring frequent removal to prevent buildups of lint around the brass sensor shoes and between the shoes and the heated cylinders. Such lint insulates and compromises the temperature sensitivity of the sensors and can produce dangerously misleading low temperature readings.
Suggested maintenance procedures include blowing lint buildups away from sensors using hand held compressed air lines before each daily startup of operation while the cylinders are cool and belts are not turning. Unfortunately, lint buildups occur more quickly and more often than can be corrected once a day, and even daily maintenance is sometimes omitted due to the pressure of busy production schedules. In other words, ironer downtime while cylinders cool to safe levels is frequently intolerable and, as a result, operators resort to short-cutting or ignoring entirely the prescribed safety procedures. Scraping tools jury-rigged to broom handles, for one example, are inserted into the operating ironer to pry lint from around clogged sensors while the ironers continue to operate, posing a serious hazard to burns and potential entanglement with moving belts. Moreover, damage to the cylinder, shoes, and particularly to the electrical leads and solder joints is risked each time cleaning of this type occurs. The alternatives, shutting down the machine for an extended period, or allowing lint deposits to compromise temperature sensing, have unacceptable commercial consequences in many instances.
Although the use of directed air flow as a dust removal mechanism is known in apparatus such as optical recorders, for example U.S. Pat. No. 5,245,601 (Hake), railway switches, U.S. Pat. No. 4,674,718 (Bjorklund), electrical contacts, U.S. Pat. No. 1,840,116 (Linton et al) and ink jet printers, U.S. Pat. No. 4,411,706 (Wallace et al), no one has heretofore proposed automatically and safely maintaining heat sensors lint-free in a hostile flatwork ironer operating environment.